The present invention relates to pay television systems. Known systems of this type may be divided into two categories. In the first, the television signal itself is encoded and the coding changed for each program. In the second category, no encoding of the television signal takes place, but the frequency of the carrier is changed so that the signal cannot be received on a particular station on the standard television receiver unless a decoder is activated. The former system requires a great deal of equipment since the timing and phase relationships must be very closely controlled. The second category requires considerably less equipment but results in very low security since a simple change of frequency will allow reception.